Photoinduced Proton Transfer of GFP-Inspired ... - ACS Publications

Subscriber access provided by UNIV OF LOUISIANA

Article

Photoinduced Proton Transfer of GFP-Inspired Fluorescent Superphotoacids: Principles and Design Cheng Chen, Liangdong Zhu, Mikhail S Baranov, Longteng Tang, Nadezhda S. Baleeva, Alexander Yu. Smirnov, Ilia V. Yampolsky, Kyril M Solntsev, and Chong Fang J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.9b03201 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 10, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 68 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Article for J. Phys. Chem. B (2019)

Photoinduced Proton Transfer of GFP-Inspired Fluorescent Superphotoacids: Principles and Design

Cheng Chen,† Liangdong Zhu,† Mikhail S. Baranov,‡,§ Longteng Tang,† Nadezhda S. Baleeva,‡ Alexander Yu. Smirnov,‡ Ilia V. Yampolsky,‡,§ Kyril M. Solntsev,¶,ℑ and Chong Fang*,†

†

Department of Chemistry, Oregon State University, 153 Gilbert Hall, Corvallis, Oregon 97331,

United States ‡

Institute of Bioorganic Chemistry, Russian Academy of Sciences, Miklukho-Maklaya 16/10,

Moscow 117997, Russia §

Pirogov Russian National Research Medical University, Ostrovitianov 1, Moscow 117997,

Russia ¶

School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia

30332, United States ℑ

New York University Abu Dhabi, P.O. Box 129188, Abu Dhabi, United Arab Emirates

To whom correspondence should be addressed: E-mail: [email protected]

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT. Proton transfer remains one of the most fundamental processes in chemistry and biology. Superphotoacids provide an excellent platform to delineate the excited-state proton transfer (ESPT) mechanism on ultrafast timescales and enable one to precisely control photoacidity and other pertinent functionalities such as fluorescence. We modified the GFP core (p-HBDI chromophore) into two series of highly fluorescent photoacids by fluorinating the phenolic ring and conformationally locking the backbone (i.e., biomimetics). The trifluorinated derivatives, M3F and P3F, represent two strongest superphotoacids with pKa* of ‒ 5.0 and ‒ 5.5, respectively, and they can efficiently transfer a proton to organic solvents like methanol. Tunable femtosecond stimulated Raman spectroscopy (FSRS) and femtosecond transient absorption (fsTA) were employed to dissect the ESPT of M3F and P3F in methanol, particularly with structural dynamics information. By virtue of resonantly enhanced FSRS signal and global analysis of fs-TA spectra, we revealed an inhomogeneous ESPT mechanism consisting of three parallel routes following the initial small-scale proton motion and contact ion-pair formation within ~300 fs. The first route consists of ultrafast protolytic dissociation facilitated by the pre-existing, largely optimized H-bonding chain; the second route is limited by solvent reorientation that establishes a suitable H-bonding wire for proton separation; the third route is controlled by rotational diffusion that requires rotation of the anisotropically reactive photoacid in a bulky solvent with a complex H-bonding structure over larger distances. Furthermore, we provided new design principles of enhancing photoacidity in a synergistic manner: incorporating electron-withdrawing groups into proximal (often as “donor”) and distal (often as “acceptor”) ring moieties of the dissociative hydroxyl group to lower the ground-state pKa and increase the ΔpKa, respectively.

2


Page 2 of 68

Page 3 of 68 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. INTRODUCTION Photoacids represent excellent model systems to study excited-state proton transfer (ESPT), one of the most fundamental and essential steps in numerous chemical and biological processes. The name “photo” and “acid” combines two important concepts in chemistry, photoinduced and acid-base reactions, or on a more fundamental level, light-matter interactions and proton motions. This class of molecules usually possesses aromatic frameworks and exhibits an decrease in the pKa value, namely, an increase of acidity, upon photoexcitation from the electronic ground state to excited state. The photoacidic strength can be characterized and classified by the excited state pKa (pKa*) and solute (acid)-solvent interactions.1 Among this chemical group, the term “superphotoacid” defines the photoacids with a negative pKa* and their capability of transferring a proton to nonaqueous solvents, which typically involve the protic and basic organic molecules in a hydrogen (H)-bonding network.2 Due to the peculiar functionality of superphotoacids to undergo proton transfer in a wide range of solvents well beyond water, significant efforts have been devoted to understanding and manipulating photoacidity over the past few decades.1,3-13 In particular, previous studies of cyano-substituted naphthols have provided valuable insights into the rational design of superphotoacids.14 The principal consideration is that photoacidity can be enhanced by applying the electron-withdrawing groups (EWGs) at electron-rich sites to lower the energy of the conjugate base.5 For 2-naphthol, two atomic sites of C-5 and C-8 (see Scheme 1 “DCN2” for the atomic numbering) show distinctly more electron density in the first singlet excited state (S1) of the anionic form, in contrast to ground state (S0). As a result, cyano substitution at either of these two sites can lead to a greater enhancement of photoacidity than substitutions at other sites of the molecule. The pKa* values of 5- and 8-cyano-2-naphthols (5CN and 8CN) were determined to be ca. ‒ 0.8 according to the Förster equation,1,8 which are more acidic than the 6-

3



and 7-cyano-2-naphthols (6CN and 7CN). Moreover, an additive effect is present when both sites are substituted. The dicyano-substituted derivative 5,8-dicyano-2-naphthol (DCN2 in Scheme 1) exhibits significantly stronger photoacidity (pKa* = ‒ 4.5) than the monocyano derivatives, so DCN2 can undergo ESPT to various alcohols and basic solvents besides water.1,14-17

Scheme 1. Structures of four representative superphotoacids based on different molecular scaffolds with their ground- and excited-state acidities (pKa and pKa*, from Förster cycle estimate)

Alternative methods or routes exist to achieve a higher acidity and/or photoacidity for these functional molecules. Without additional EWG substitutions, a replacement of the naphthalene backbone with an electron-deficient methylquinolinium produces an interesting superphotoacid Nmethyl-6-hydroxyquinolinium (NM6HQ+, Scheme 1) with a similar photoacidic strength (pKa* ≈ ‒ 4.0)7 as DCN2. We note that subsequent studies adopted direct kinetic measurements, extensive Brownian dynamics simulations and Spherically Symmetric Diffusion Problem (SSDP) calculations with the adapted force field, which led to the determination of pKa*≈ ‒7.0 for NM6HQ+ with non-associating counterions in water, and its ESPT activation energy is smaller than that of DCN2 in water.18,19 Another pertinent example is the modification of 8hydroxypyrene-1,3,6-trisulfonate (pyranine or HPTS), a well-investigated photoacid.3,4,6,20,21 By

4


Page 4 of 68

Page 5 of 68 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


converting three side-chain sulfonic acid groups to the more electron-withdrawing sulfonamides and sulfonic esters, the photoacidity can be increased by ca. 2–5 pKa units from the parent HPTS (pKa*≈0.4).22 The strongest photoacid synthesized in this substitution series is tris(1,1,1,3,3,3hexafluoropropan-2-yl)-8-hydroxypyrene-1,3,6-trisulfonate (HPTS-HFP, Scheme 1) with a pKa* of ‒ 3.9.10 In addition, the cyanine-based photoacids have recently drawn great attention due to their extraordinary photoacidities as well as the non-fluorescent characters.23 Quinone cyanine 9 (QCy9) has been reported to be the strongest photoacid to date with the ESPT rate of ~1×1013 s–1 and an incredible low pKa* of ‒ 8.5, which essentially correlate with a barrierless ESPT reaction from QCy9 to water, methanol, and ethanol with the same rate.24 Among these synthesized and engineered superphotoacids, intramolecular charge transfer (ICT) has been identified as an important feature for the energy lowering of excited states.5,25,26 This stabilization effect is more pronounced for the deprotonated form (PB) than for the protonated form (PA) of the photoacid, hence leading to a more exergonic/exothermic PT reaction and a consequent lower pKa*. The aforementioned strengthened photoacids all adopt the “donor-acceptor” structures with an enhanced ICT character, which is achieved by placing strong EWGs at the acceptor moieties that are typically located at the distal ring(s) from the original hydroxylated ring. The electrophilic substitutions at more electronegative sites of the PB form would be expected to achieve stronger photoacidity due to a better dispersed and delocalized electron density with respect to substituted cases at those less electronegative sites. This is supported by the fact that, in the excited state, 5CN and 8CN are more acidic than 6CN and 7CN among the monocyano naphthols (see Scheme 1 for the atomic numbering).5,16 For the multi-ring systems like naphthol and hydroxypyrene, the proximal ring bound to the hydroxyl group is conventionally considered as the donor moiety. It is noteworthy that the systematic study of substituent and site effect on

5



photoacidity upon substitution at the donor ring remains to date experimentally underexplored.27 For instance, the 1- and 3-cyano-2-naphthols (1CN and 3CN) have not yet been synthesized to further examine the ICT principle despite semiempirical calculations that predicted 3CN to be a strong photoacid.25 HPTS and its derivatives have EWGs at both proximal and distal rings which, however, complicates the elucidation of effect of clearly isolated proximal-ring substitutions.10,22 In this work, we designed two series of photoacids and comprehensively characterized the most photoacidic compound of each series using a number of advanced spectroscopic techniques, which enrich the understanding of fundamental determinants of photoacidity as well as the excited-state structural behaviors of superphotoacids. The design of these photoacids has been inspired by the photochemical properties of green fluorescent protein (GFP) which has revolutionized molecular and cellular biology for decades.28-31 It is known that the wild-type GFP emits green light upon ultraviolet irradiation because the core chromophore undergoes ESPT on ultrafast timescales and converts to a deprotonated intermediate I*.32-34 This transient process is highly energy-conserving, indicated by the high fluorescence quantum yield (FQY, Φ = 0.79) of wild-type GFP upon excitation of the neutral form of the embedded chromophore. However, the synthetic GFP chromophore 4’-hydroxybenzylidene-1,2-dimethyl-imidazolinone (p-HBDI, Scheme 2) loses not only high FQY but also the ESPT capability in solution phase.35 This observation reveals the important role of a restrictive protein matrix in functionalizing the chromophore.31 In contrast to a rigid protein environment, ring twisting motions around the bridge bond within p-HBDI are allowed and become favorable in solution, therefore opening up nonradiative decay pathways and leading to a very low FQY of ~10‒4, essentially deeming it non-fluorescent at room temperature.8,35 Interestingly with a relatively small molecular footprint, p-HBDI is not capable of ESPT in aqueous solution. This is mainly due to the following reasons. (1) Altered electronic structure

6


Page 6 of 68

Page 7 of 68 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


under flexible conformations intrinsically weakens photoacidity. This is evinced by a large redshift in absorption and emission peak of both the neutral and anionic p-HBDI from solution to protein pocket. It indicates better stabilization of the excited state in a more rigid nuclear framework by protein matrix. The ground-state pKa was determined to be ~8.2 in aqueous solution36 while ~6 or lower in GFP with various mutations.37 The latter is in good agreement with theoretical pKa calculations of a coordinate-frozen p-HBDI (~6.4 estimated from the Förster equation),13 which suggest that the entropic contributions due to internal rotational modes (not the surrounding residues of the protein host per se) mainly explain the pKa increase from protein to solution in a ground-state deprotonation reaction.38 As a result, p-HBDI has a higher pKa* in solution according to the Förster cycle. (2) The short excited-state lifetime of neutral p-HBDI limits the ESPT efficiency. In essence, there exists an intrinsic competition between different excited-state energy dissipation pathways, e.g., ESPT, internal conversion, and fluorescence. The pKa* of p-HBDI is estimated to be 2.1,13,36,39 corresponding to a very slow ESPT rate. Therefore, the

Photoinduced Proton Transfer of GFP-Inspired ... - ACS Publications

Recommend Documents